This invention relates generally to television apparatus and, more particularly, to apparatus for the separation of the luminance and chrominance components of an NTSC color television signal.
During the past few years there has been an increasing use of delay-line comb filters for the separation of the luminance and chrominance components of an NTSC encoded signal. This technique, capable of producing full band luminance output to the limit of the input signal bandwidth, is based upon the assumption that above a frequency of about 2.5 MHz negligible video changes in the vertical picture plane occur over any three successive television lines. When that assumption fails, as it frequently does with program video, certain picture anomalies occur as a result of imperfect cancellation of chrominance in the luminance output signal, and vice versa. The condition of spurious chroma in the luminance output has been observed to be the more serious problem as it gives rise to objectionable scintillating serrations along horizontal edges of surfaces where sudden hue or saturation changes exist. When a delay-line comb filter is used for freeze-frame display, this edge effect becomes visible as a 15Hz edge flicker because of the frame-rate inversions of chrominance required for that mode of operation.
Both manifestations can be substantially minimized if the uncancelled or spurious chrominance can be deleted from the luminance signal. A known system for accomplishing this utilizes a detection circuit capable of determining when chrominance transitions are occurring in the vertical direction, and when such transitions are detected a gating signal is generated which causes the luminance signal to be suitably filtered to eliminate the spurious chrominance that would otherwise have appeared in the luminance signal. The gated filtering of the chrominance is controlled by a manually adjustable threshold setting so that chrominance deletion can be utilized at as low a chrominance differential level as is permitted by the signal-to-noise ratio. This system implemented in the analog domain, has operated satisfactorily in actual video operations. However, since in this system the 3.58 MHz luminance signal is subjected to low-pass filtering after the spurious chrominance has occurred, there is a possibility of luminance detail being obscured or eliminated in the filtered signal so as to degrade the reconstructed television picture when the filtered luminance and chrominance components are recombined in the color decoder.
Although delay-line comb filters implemented in the analog domain for separation of the luminance and chrominance components of an NTSC color television signal have been developed to a high standard of performance, it is generally recognized that in the future an increasing percentage of television broadcasting plant processing will be performed digitally. At present, color encoding and decoding in the digital domain are relatively costly operations because arithmetic operations must be performed at high speeds that tax the capability of existing logic hardware. However, as digital circuits and stores become faster and less expensive, the concept of digital processing becomes increasingly practical and attractive.
A recent development in digital television equipment is a device known as the "Electronic Still Store" wherein an NTSC color television signal is generated from a magnetically stored pulse code modulated (PCM) video frame. Because of the characteristics of the NTSC color television signal, in order to generate a correct signal from a single stored frame, it is necessary to separate the luminance and chrominance components, invert the chrominance component on alternate frames, and then recombine the two signals. Care must therefore be taken to avoid any luminance energy from getting into the chrominance channel where it will undergo an inversion on alternate frames and generate a disturbing 15 Hz flicker in the reconstructed television picture. It is important, therefore, that a clean separation of chrominance and luminance components be performed.
As has been noted above, separation of the luminance and chrominance components of an NTSC color television signal is best accomplished by comb filtering techniques, which take advantage of the frequency relationship between the horizontal line rate and the color subcarrier signal. A typical comb filter utilizes three adjacent television lines in a given field and selectively adds and subtracts them to obtain the chrominance and luminance signals. If three sequential television lines are labeled "Top" (T), "Middle" (M) and "Bottom" (B), a comb filter using one of the following algorithms may be used to separate chrominance (C) and luminance (Y) within the chrominance passband: EQU C = 1/2[M - 1/2(T+B)] and Y = 1/2[ M + 1/2(T+B)] [1] EQU c = 1/2(m-t) and Y = 1/2(M+T) [2] EQU c = 1/2(m-b) and Y = 1/2(M+B) [3]
an examination of the function of these algorithms reveals that the comb filter effectively operates by sampling and averaging, with particular weighting coefficients, three picture elements from three adjacent lines in the case of algorithm (1), and two picture elements from two adjacent lines in the case of algorithms (2) and (3). Algorithm (1) is desirably used whenever the chrominance or the luminance on the television lines being combined are reasonably constant. However, if the television picture should contain a luminance or a color transition in the vertical direction, algorithm (1) will generate a disturbing transient in the reconstructed television picture. Accordingly, there is a need for an automatic adaptive comb filter capable of detecting such vertical transitions and in response thereto to utilize algorithm (2) or (3), or to bypass the comb filter, to minimize the transient distortions.
The primary object of the present invention is to provide in comb filter apparatus a solution to this need, thereby to improve the quality of television displays.